The key targets that determine the activation versus the induction of p53 are still poorly understood. These studies will define the critical networks of interactions that together determine p53 transcriptional activation. Our objective is to manipulate these networks to develop new rational targeted p53 cancer therapies. To achieve this we are using adenovirus infection as a powerful but simple genetic system to define the p53 growth regulatory network, as well as a lytic agent for p53 cancer therapy. Adenovirus E1B-55K is required for p53 degradation in viral infection. Thus, an E1B-55K mutant adenovirus, ONYX-015, induces high levels of nuclear p53. This was expected to prevent viral replication in normal cells, but not in p53-mutant tumor cells. On this premise, ONYX-015 was tested in clinical trials as a p53 cancer therapy. However, we discovered that although ONYX-015 induces very high p53 levels, as expected, p53 activated transcription was suppressed. This leads to a fundamental question-how are high levels of p53 inactivated? To address this, we have now shown that p53 activated transcription is suppressed in ONYX-015 infected cells, even upon treatment with adriamycin, irradiation or Mdm2 antagonists. Using a genetic approach, we have discovered that there is another viral protein, E4-ORF3, which prevents p53 transcriptional activation independently of E1B-55K. In Aim 1 we will exploit E1B-55K/E4-ORF3 mutant viruses for a powerful comparative proteomics approach to define critical p53 interacting complexes that distinguish (and likely determine) activation versus induction of p53 in response to adriamycin and adenoviral replication. Novel p53-interacting proteins have already been identified and our preliminary data indicate that they are critical determinants of p53 activation. RNAi and cDNA expression experiments will be used to define their role in determining p53 activation and growth regulation/survival. These studies will define the hierarchy of critical p53 interactions that determine p53 activation versus repression in response to genotoxic and oncogenic stress. In Aim 2, we will look upstream to the viral protein protagonist, E4-ORF3. We will test the hypothesis that E4-ORF3 subverts key DNA damage signals and downstream phosphorylation targets to prevent p53 transcriptional activation in ONYX-015 infected cells. In Aim 3 we will determine if chromatin modifications, transcriptional initiation or elongation are subverted to prevent the activation of p53 target promoters in reponse to genotoxic and oncogenic stress. These studies will integrate the critical upstream signals, downstream protein interactions/modifications with the specific transcriptional activation of p53 effectors. We will define a critical new mechanism whereby p53 is inactivated in adenovirus infection, which will change a central dogma. These new mechanistic insights will enable the rational development of potent p53 selective oncolytic viruses and non-genotoxic drugs that activate p53 transcription. p53 was first discovered with a DNA viral protein; these studies will exploit viral infection to define the critical networks of p53 interactions and how to uncouple them for cancer therapy.